Tunable capacitor

ABSTRACT

A tunable capacitor includes a first electrode and a second electrode, each being formed of a conductive material. The tunable capacitor further includes a third electrode between the first electrode and the second electrode, and a dielectric material interposed between the first electrode and the third electrode, and between the second electrode and the third electrode. The third electrode is movable relative to the first electrode and the second electrode by a stepper motor, to adjust and tune a capacitance of the tunable capacitor.

BACKGROUND

This document describes a tunable capacitor, and more particularly toproduction of electrical components for electrical circuits,specifically for precision Radio Frequency (RF) applications.

A capacitor is a device for storing electrical energy. The amount ofstored energy is defined as a capacitance of the capacitor, which ismeasured in units of Farads. Some capacitors can be tuned, i.e. having avariable capacitance, but adjustable to a particular capacitance. Suchtunable capacitors are sometimes referred as variable capacitors,trimmer-capacitors, or simply “trimmers”.

Trimmers come in a variety of sizes and levels of precision. Thecapacitance of trimmers can be adjusted with a small screwdriver, inwhich several turns of an adjustment screw can reach a desired endvalue, allowing for some degree of accuracy. Conventional trimmersinclude two electrically conductive electrodes separated by a dielectricmaterial, and the distance between the electrodes and/or dielectricmaterial affects the capacitance. To tune a trimmer, the distancebetween the electrodes or overlapping area of the electrodes is changed,and results in changing the capacitor's capacitance. The followingformula governs such changes:

$\begin{matrix}{{C = \frac{ɛ \cdot S}{d}},} & (1)\end{matrix}$

where

-   -   C—capacitance of the trimmer,    -   ∈—dielectric constant of dielectric,    -   S—overlapping area,    -   d—distance between the electrodes

Conventional trimmers, however, are not very accurate, and have limitedrange of capacitance value. Further, they do not allow automatic digitalcontrol of the capacitance value with high accuracy, as is required forsuch applications as tunable RF filters.

SUMMARY

This document presents a tunable capacitor that overcomes thelimitations of conventional tunable capacitors and trimmers. The tunablecapacitor of the present disclosure is highly accurate, provides a largerange of capacitance value, and allows for automatic digital control ofthe capacitance value. Further, the tunable capacitor described hereinhas high power handling capability.

In some implementations, a tunable capacitor is embodied as amechanically tunable trimmer, in which a capacitance of the tunablecapacitor can be adjusted or tuned by means of an external control. Theexternal control can be a mechanical driver powered by a stepper motor.In preferred instances, the stepper motor motion is controlled digitallyfrom a computer in communication with the stepper motor.

In one aspect, a tunable capacitor is disclosed. The tunable capacitorincludes a first electrode and a second electrode, wherein each of thefirst and second electrodes are formed of a conductive material. Thetunable capacitor further includes a third electrode between the firstelectrode and the second electrode. The tunable capacitor furtherincludes a dielectric material interposed between the first electrodeand the third electrode, and between the second electrode and the thirdelectrode. The third electrode is movable relative to the firstelectrode and the second electrode by a stepper motor, to adjust andtune a capacitance of the tunable capacitor.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other features and advantages willbe apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects will now be described in detail with referenceto the following drawings.

FIG. 1 illustrates a tunable capacitor in accordance withimplementations of the present disclosure.

FIGS. 2A-2C illustrate various mechanisms for implementing a tunablecapacitor.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

This document describes a tunable capacitor, and more particularly amechanically tunable capacitor having high accuracy in the designedrange. Further, the tunable capacitor described herein provides a largerange of capacitance value and allows for automatic digital control ofthe capacitance value.

In accordance with some implementations, as shown in FIG. 1, a tunablecapacitor 100 includes two fixed electrodes 1 and 2, and a slidingelectrode 3 provided between the two fixed electrodes 1 and 2. Thetunable capacitor 100 is equivalent to two variable capacitors connectedin series. When the sliding electrode 3 is in a lowest position, i.e.furthest displaced from the two fixed electrodes 1 and 2 as shown inFIG. 1, then the capacitance is minimal. However, when the slidingelectrode 3 is in a highest position, i.e. most overlapping with the twofixed electrodes 1 and 2, the capacitance is maximal.

Depending on the relative position of the electrodes, the tunablecapacitor 100 provides capacitance for certain values within aparticular designed range. The sliding electrode 3 is attached to astepper-motor that moves the sliding electrode 3 between the electrodes1 and 2, without touching them. The gap between the first fixedelectrode 1 and the sliding electrode 3, and between the second fixedelectrode 2 and the sliding electrode 3 may be air or filled with any RFdielectric, such as Teflon, or other suitable material.

The high accuracy provided by the tunable capacitor 100 is provided bythe fixed (not movable) capacitor plates 1 and 2, contrary to othertechnologies where one or two capacitor plates are movable. The slidingelectrode 3 is movable, and is not electrically connected to any circuit(or ground); it is an electrically isolated electrode, which is easierto move without compromising electrical performance.

Further, the gaps between the electrodes need not be kept constant forhigher accuracy, as is the case for some conventional capacitors.Assuming that the central electrode deviates from its central positionto one side, the gap between one of the fixed electrodes 1 or 2 and thesliding electrode 3 is decreased. Accordingly, this results in increasedcapacitance, according to the formula (1). Concurrently, the gap betweenthe central sliding electrode 3 and the other fixed electrode 2 or 1 isincreased, which results in decreased capacitance, according to formula(1). Thus, due to the series connection of the two capacitivearrangements, created by the two gaps as shown in FIG. 1, the totalcapacitance remains substantially unchanged. The fixed electrodes 1 and2, and the distance between the sliding electrode 3, compensate eachother as shown in formula (2):

$\begin{matrix}{{\frac{1}{C_{tot}} = {\frac{1}{C_{1}} + \frac{1}{C_{2}}}},} & (2)\end{matrix}$where: C_(tot) is the total capacitance of the tunable capacitor,

-   -   C₁ is the capacitance between the central electrode (3) and the        side electrode (1)    -   C₂ is the capacitance between the central electrode (3) and the        side electrode (2)

In other implementations, a tunable capacitor 200 includes a fixedelectrode 100 and two movable electrodes 200, which are movable to sliderelative to the fixed electrode 100. As shown in FIGS. 2A-2C, the fixedelectrode 100 is fixed by any fixing mechanism.

FIGS. 2A-2C show a tunable capacitor 200 in which movable electrodes 200are connected together by a traverse 300. The traverse 300 is preferablyformed of a non-conductive material. The traverse 300 is connected toboth movable electrodes 200 and preferably aligns and spaces the movableelectrodes 200 relative to the fixed electrode 100. The movableelectrodes 200 are movable according to any number of moving mechanisms,the preferred of which are described below.

FIG. 2A shows a tunable capacitor 200 that includes a threaded nut 4which receives and cooperates with threaded screw 5, which is turned andcontrolled by stepper motor 6. The stepper motor 6 can be controlled viaelectrical terminals 7, which can supply electrical pulses from acomputer controller to the stepper motor 6. The electrical pulses caninclude a control signal to turn the threaded screw 5 clock-wise orcounter clock-wise, to move the movable electrodes 200 closer over thefixed electrode 100 or away from the fixed electrode 100, respectively.

FIG. 2B shows the tunable capacitor 200, which includes a linearactuator 9 to control a push-pull rod 8, to push the movable electrodes200 closer over the fixed electrode 100 or pull the movable electrodes200 away from the fixed electrode 100, respectively. The linear actuator9 can be controlled via electrical terminals 10, which can supplyelectrical pulses from a computer controller to linear actuator 9. Theelectrical pulses can include a control signal to incrementally push outor pull back the push-pull rod 8. FIG. 2C shows a tunable capacitor 200in which a push-pull rod 13 is controlled by magnet 11, around which acoil 12 is wound. Direct current signals form an external source, suchas a computer or other logical controller, controls a magnetic forceexerted on the push-pull rod 13. These implementations provide accuracyof precision motion and do not require a high control voltage like manyconventional trimmers.

In preferred implementations, control voltage terminals and the RFsignal terminals are separated, which does not require a DC blockcircuit. As a result, the quality of the tunable capacitor is muchhigher than conventional designs. In addition, the tunable capacitordescribed herein, especially as shown in FIGS. 2A and 2B, has no springsand is insensitive to vibration. The capacitive value does not depend ona position of the tunable components, and therefore any error iseliminated.

The tunable capacitor can handle high power, and has a dielectricstrength to be able to withstand 1000 volts or more. In preferredimplementations, the tunable capacitor uses an aluminum oxide, or“alumina” dielectric having a dielectric constant of approximately 9.5.Other dielectric materials can be suitably used, such aspolytetrafluoroethylene, otherwise known as Teflon®, for example.Referring back to the exemplary implementation shown in FIG. 2C, forexample, a gap between electrodes 1 and 2 requires a dielectricthickness of approximately 0.010″, and the dimensions of electrodes areapproximately 0.400″×0.200″. Accordingly, the overall dimensions of thecapacitor is 0.400″×0.400″×0.100″. Of course, these dimensions areexemplary, and actual dimensions could vary by up to 10% or more fromthose disclosed.

The tunable capacitor described herein that uses alumina dielectric canwithstand up to 1055V or more, while a tunable capacitor using a Teflondielectric can withstand up to 4700V or more. Accordingly, the tunablecapacitor described herein can withstand high power as well.

Referring to FIG. 2C as an example, the maximum capacitance is achievedwhen the electrodes 2 are in the most left position overlapping theelectrode 1 completely, and can be calculated as follows:

$\begin{matrix}{{{C\lbrack{pF}\rbrack} = \frac{0.2249*\varepsilon*{S\left\lbrack {{sq}.\mspace{14mu}{in}.} \right\rbrack}}{2*{D\lbrack{in}\rbrack}}},} & (2)\end{matrix}$Where: C is the capacitance in Pico farads;

-   -   ∈ is dielectric constant of the capacitor dielectric, (i.e. 9.5        for Alumina, 2.1 for Teflon);    -   S is the area of the electrode 2 in squared inches;    -   D is the gap between electrodes 1 and 2 in inches.

This formula (2) results in maximum capacitance value of approximately8.5 pF, which is sufficient for an RF application. The minimum value isclose to 0 pF.

The break-down voltage for the tunable capacitor in which alumina isused for the dielectric can be given as:

${V = {{{13400\left\lbrack \frac{V}{mm} \right\rbrack}*2*0.010^{''}*3.937} = {1055\lbrack V\rbrack}}},$

For a Teflon dielectric, the break-down voltage is even higher, around4700[V]. The reactive power stored in the capacitor, then, can becalculated using the following formula:

$\begin{matrix}{{P = {\frac{C \cdot \cdot V^{2}}{2} = {\frac{8.5 \cdot 10^{- 12} \cdot 1055^{2}}{2} = {4.7\lbrack{\mu W}\rbrack}}}},} & (3)\end{matrix}$

Dissipating power of a tunable capacitor with Q=500 due to imperfectmaterials is:

$P_{dis} = {\frac{P}{Q} = {\frac{4.7}{500} = {9.5\lbrack{nW}\rbrack}}}$

This is a very small power, and cannot damage the tunable capacitor.However, as described above, power is not the damaging factor; thevoltage is. The tunable capacitor can withstand 1055V with alumina and4700V with Teflon dielectric. Accordingly, the tunable capacitor canwithstand high power as well, the threshold of which can be estimatedonly for a particular application in which the capacitor is used.

Although a few embodiments have been described in detail above, othermodifications are possible. Other embodiments may be within the scope ofthe following claims.

The invention claimed is:
 1. A tunable capacitor comprising: a firstelectrode and a second electrode, the first and second electrodes beingformed of a conductive material; a third electrode between the firstelectrode and the second electrode; and a dielectric material interposedbetween the first electrode and the third electrode, and between thesecond electrode and the third electrode, the third electrode beingmovable relative to the first electrode and the second electrode by astepper motor to adjust and tune a capacitance of the tunable capacitor.2. The tunable capacitor in accordance with claim 1, wherein thedielectric material includes aluminum oxide.
 3. The tunable capacitor inaccordance with claim 1, wherein the dielectric material includespolytetrafluoroethylene.
 4. The tunable capacitor in accordance withclaim 1, wherein a thickness of the dialectric material is approximately0.010 inches.
 5. The tunable capacitor in accordance with claim 1,wherein a length and a width of each of the first, second and thirdelectrodes is approximately 0.400 inches by 0.200 inches, respectively.6. A tunable capacitor comprising: a first electrode and a secondelectrode, the first and second electrodes being formed of a conductivematerial; a third electrode between the first electrode and the secondelectrode, the third electrode being formed of a conductive material;and a dielectric material interposed between the first electrode and thethird electrode, and between the second electrode and the thirdelectrode, the first and second electrode being selectively movable by astepper motor relative to the third electrode to adjust and tune acapacitance of the tunable capacitor.
 7. The tunable capacitor inaccordance with claim 6, wherein the dielectric material includesaluminum oxide.
 8. The tunable capacitor in accordance with claim 6,wherein the dielectric material includes polytetrafluoroethylene.
 9. Thetunable capacitor in accordance with claim 6, wherein a thickness of thedialectric material is approximately 0.010 inches.
 10. The tunablecapacitor in accordance with claim 6, wherein a length and a width ofeach of the first, second and third electrodes is approximately 0.400inches by 0.200 inches, respectively.